The TeacherGeek 40 Piece Multi-Color Pulley Set contains ten each of the following sizes: 70mm, 55.5mm, 25.5mm, 9mm . These are used to create pulley driven transmissions such as for vehicles, wind turbines, lifts and pumps. Splined holes hold the dowels for a secure fit. Also the off center holes may be used to incorporate cams into the design. The unique pulley design allows for non-slip grip and easy exchange of rubber bands.